4 Answers
4

Natural 20: If you roll a 20 on the die when making an attack roll, you score a critical hit if your total attack roll is high enough to hit your target’s defense. If your attack roll is too low to score a critical hit, you still hit automatically.

Precision: Some class features and powers allow you to score a critical hit when you roll numbers other than 20 (only a natural 20 is an automatic hit).

For your example, if the to hit roll was a natural 20 AND the modified number would still hit the target's defense then the result is a critical.

The rules have pretty much always read that a critical roll is a critical, despite any modifications to the result after the roll. Reducing the roll by a d6 is modifying the end result, not the fact that they rolled a critical hit. The hit is still critical, but the d6 subtraction may be useful for other things.

The roll itself still stands. You can never change a physical roll, but you can always add modifiers. Unless a power specifically says "Negates a critical attack" (And there are a couple that do, I think, would have to pull out the books to be sure) then it can only change the end result, not stop a natural 20 from being a crit.
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RGThigpenDec 21 '11 at 15:34

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@RGThigpen there are several things that negate crits (fortification armor gives you a roll, there is a feat that does something similar, some other interrupt powers). The case here is wondering whether this particular power (or powers like it) alter the number on the die or the result. This power in the first case could be used to achieve a crit or negate one, in the second case it could just give you a chance to change a hit to a miss or a miss to a hit...
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wax eagle♦Dec 21 '11 at 15:39

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Unless the power says otherwise (like the armor and feats you mentioned, yes) then it is only a modifier, not a change to the physical roll.
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RGThigpenDec 21 '11 at 15:42

A natural 20 will always hit regardless of modifications to the roll. However, if the d6 used happened to drop the total attack roll enough to not meet the target defense it is resolved as a normal hit and does not deal max damage.
This is stated in the player's handbook pg 278 and rules compendium pg 216.

Specific always trumps general. Yes, the rules state that a natural 20 is an auto-hit. That's a general, sweeping statement. For example, I could roll a natural 20 throwing a feather at a dragon from 5 feet away, but that doesn't mean it's going to be a damaging hit and critical.

Logic and common sense need to be utilized.

To that end, a Luckbender modifies the roll, not the result. Yes it achieves the same thing, but the skill specifically says that it modifies the triggering role, which is pretty straightforward. If the target rolls a natural 20, the Luckbender can use his ability to change the roll to whatever he gets on his 1D6. This removes the condition of always hit because the die roll is no longer a natural 20. This is also different from condition effects like blind. Rather than a condition in which the 20 signifies a lucky break, the Luckbender manipulates the universe to alter how things could have been. Think of him like a really, really limited Jedi. He can see the crit coming, so he used his ability to avoid the critical strike. He might even have been lucky enough to avoid the entire hit.

Anyways, as per the rules, specific trumps general.

Text from the ability:

Trigger: You, an ally, or an enemy in the burst makes an attack roll, skill check, or saving throw.

Effect: Roll 1d6, and either add or subtract the result from the triggering roll.

You aren't modifying the die though. Your modifying the roll which is the result of the die and modifiers. Luckbender is basically just another modifier. A crit is still a crit with it.
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wax eagle♦Jun 3 at 9:49

Again, specific trumps general. The luckbender ability is very specific as to its effect, and the automatic success on a 20 is a general rule. I already gave you a scenario where the auto-hit 20 would not achieve a critical hit. And yes, it was deliberately ridiculous to demonstrate that 20 is not always a guaranteed success.
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Lino Frank CiaralliJun 3 at 16:01

your example does in fact lead to a critical in 4e. Crits aren't crits in 4e only if you don't meet the target defense with the roll. There's nothing in the rules that allows for that kind of interpretation. And 4e is a pretty tightly defined combat system. A feather may only do 1 point of damage (or none, as it's not heavy enough to be an improvised weapon), but it'd still register a crit if it hit on a natural 20. The general rule isn't trumped here as the actual die (which is what triggers the crit) is not modified.
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wax eagle♦Jun 3 at 16:05

You literally just said a feather could critically hit a dragon. I have nothing with which to respond to that leap of logic. The effect specifically states the roll is modified, which means a roll of 20 is NOT a 20. It can't be more straightforward than what it says.
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Lino Frank CiaralliJun 3 at 16:09

are you familiar with 4e? A roll (Specifically, an attack roll) is defined as the d20+modifiers. A modifier to the roll does not affect what the d20 says.
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wax eagle♦Jun 3 at 16:23